Optical pickup and optical storage device

ABSTRACT

An optical pickup for an optical storage device includes a stem, a substrate mounted on the stem, and a laser diode mounted on the substrate. The substrate is integrally formed with an optical signal detector and an error signal detector for focusing error detection and tracking error detection. A cap is mounted on the stem so as to accommodate the substrate and the laser diode. A beam splitter unit including a polarization beam splitter and a beam splitting element is mounted on the cap. A hologram for diffracting a reflected beam toward the error signal detector is interposed between the cap and the beam splitter unit. The substrate is biased at a given potential, and has an insulating film opposed to the stem.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup (optical head) for anoptical device such as an optical disk drive, optical card drive,optical scanner, and microscope unit.

2. Description of the Related Art

An optical disk inclusive of a magneto-optical disk has receivedattention as a memory medium that becomes a core in the recent rapiddevelopment of multimedia, and it is usually accommodated in a cartridgecase to be provided as an optical disk cartridge for practical use. Theoptical disk cartridge is loaded into an optical disk drive to performreading/writing of data from/to the optical disk by means of an opticalpickup.

The optical pickup in a recent optical disk drive intended to realizesize reduction is composed of a fixed optical assembly and a movableoptical assembly, wherein the fixed optical assembly includes a laserdiode, a beam splitter for reflecting and transmitting a laser beam, anda photodetector for receiving reflected light from an optical disk,whereas the movable optical assembly includes an actuator having acarriage and an objective lens mounted on the carriage. The carriage ismovable in the radial direction of the optical disk along a pair ofrails by means of a voice coil motor (VCM).

A write-power laser beam output from the laser diode of the fixedoptical assembly is first collimated by a collimator lens, nexttransmitted by the beam splitter, next reflected by a beam raisingmirror of the actuator, and finally focused on the optical disk by theobjective lens, thereby writing data onto the optical disk. On the otherhand, data reading is performed by directing a read-power laser beamonto the optical disk. That is, reflected light from the optical disk isfirst collimated by the objective lens, next reflected by the beamsplitter of the fixed optical assembly, and finally detected by thephotodetector, thereby converting the detected optical signal into anelectrical signal.

In general, recording media such as an optical disk and amagneto-optical disk are exchanged for use with an optical disk drive.Further, these recording media have warpage or undulation due to strainin forming the media, resulting in the tendency of eccentricity orinclination of the recording media. Accordingly, focusing errordetection and tracking error detection must be carried out to read outinformation recorded on the recording media. A conventional opticalpickup for a magneto-optical disk employs many optical componentsincluding a plurality of lenses and a plurality of polarization beamsplitters, so as to perform the detection of information recorded on themagneto-optical disk and also perform focusing error detection andtracking error detection.

U.S. Pat. No. 5,708,644 discloses an optical pickup using a beamsplitter unit having a polarization beam splitter and a beam splittingelement to reduce the size of an optical system. In this U.S. patent, ahologram for separating off a focusing error signal and a tracking errorsignal from a reflected beam is mounted on the lower surface of the beamsplitter unit. Further, a laser diode, a first photodiode for detectinga magneto-optical signal, a second photodiode for detecting the focusingerror signal, and a third photodiode for detecting the tracking errorsignal are mounted on a stem.

Thus, the first, second, and third photodiodes are mounted on the stemin the optical pickup described in the above U.S. patent, so that thereis a problem of insufficient integration of the photodiodes. To solvethis problem, it is considered to provide a silicon (Si) substrateintegrally formed with these photodiodes.

In an optical pickup for a magneto-optical disk, a PIN-photodiode isgenerally used as each photodiode to meet the requirement for ahigh-speed response signal. Accordingly, in the case of forming aPIN-photodiode integrally with an Si substrate, a reverse bias voltageis applied to the Si substrate to increase a response speed as aphotodetecting element. The application of a reverse bias voltage meansapplying a bias voltage to the cathode of the photodiode.

The stem (optical base) on which the Si substrate is mounted is bondedto a drive base, so that the stem is at the same potential as a groundpotential. Therefore, the lower surface of the Si substrate must beinsulated from the stem, so as to apply a reverse bias voltage to the Sisubstrate. Further, the laser diode chip is mounted on the upper surfaceof the Si substrate, so that an insulating layer must be interposedbetween the lower surface (electrode surface) of the laser diode chipand the upper surface of the Si substrate. However, in the case thatthere is a potential difference between the reverse-biased Si substrateand the electrode surface of the laser diode chip, especially in thecase that there are high-frequency variations in potential, thepotential of the Si substrate is influenced by variations in potentialof the electrode of the laser diode.

Such high-frequency variations in potential occur especially in writingdata, and have adverse effects on a photodiode for detection of afocusing error signal, a photodiode for detection of a tracking errorsignal, and a photodiode for monitoring an output from the laser diode,thus causing instability in detecting signals output from thesephotodiodes.

A region on the Si substrate except the photodiodes (photodetectingregions) also has sensitivity to light, and generates electrical chargewhen receiving light. This electrical charge has an influence on signalcurrents generated in the photodiode regions, causing a problem thathigh-quality signal currents cannot be obtained. This is due to the factthat all of the light quantities of the laser beam output from the laserdiode cannot be transmitted or reflected by each optical component, buta part of the laser beam remains in the optical unit to become straylight.

This stray light may enter the photodiode for detection of amagneto-optical signal, the photodiode for detection of a focusing errorsignal, the photodiode for detection of a tracking error signal, and aphotodiode for automatic power control (APC), causing adverse effects onsignal currents. As a known technique for shielding such stray light, ametal film is provided on the entire surface of the substrate except thephotodiode regions. The metal film is usually formed of aluminum commonto the material of wiring on the substrate. However, reflected lightfrom the optical components is further reflected by the metal film toresult in an increase in stray light.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalpickup in which an optical signal detecting mechanism is compact andintegrated, and the quality of a signal detected by a photodetector isimproved.

It is another object of the present invention to provide an opticalstorage device including an optical pickup in which the quality of asignal detected by a photodetector is improved and low-cost productionis allowed.

It is a further object of the present invention to provide an opticalpickup and an optical storage device which can solve the problem ofstray light incident on the photodiode regions to improve the quality ofa regenerative signal and the quality of a servo signal.

In accordance with an aspect of the present invention, there is providedan optical pickup comprising a stem; a substrate mounted on the stem; alaser diode mounted on the substrate; and a photodetector provided onthe substrate for detecting return light from an object to beirradiated; the substrate being biased at a given voltage and having aninsulating member opposed to the stem.

Preferably, the photodetector comprises a first photodetector fordetecting a regenerative signal, a second photodetector for detecting aservo signal from a laser beam focused on the object, and a thirdphotodetector for monitoring an output from the laser diode. The opticalpickup further comprises a beam splitter unit having a polarization beamsplitter and a beam splitting element formed of a birefringent crystal.The beam splitter unit further has a hologram lens for focusing monitorlight to the third photodetector.

Preferably, each of the first and second photodetectors comprises aPIN-photodiode. The laser diode has a first electrode opposed to thesubstrate with a first insulating film, a conductor film, and a secondinsulating film being interposed between the first electrode and thesubstrate. Preferably, the substrate comprises an Si substrate, and eachof the insulating member, the first insulating film, and the secondinsulating film comprises an SiO₂ film. Preferably, the conductor filmand the stem are connected by a first wire. The laser diode further hasa second electrode, and the first and second electrodes of the laserdiode are connected to the stem by second and third wires, respectively.

In accordance with another aspect of the present invention, there isprovided an optical storage device capable of at least readinginformation stored in an optical storage medium, comprising a base; acarriage movable along the optical storage medium; a stem mounted on thebase; a substrate mounted on the stem; a laser diode mounted on thesubstrate; an objective lens mounted on the carriage for focusing alaser beam from the laser diode onto the optical storage medium; and aphotodetector provided on the substrate for detecting at least aregenerative signal from a reflected beam from the optical storagemedium; the substrate being biased at a given potential and having aninsulating member opposed to the stem.

In accordance with a further aspect of the present invention, there isprovided an optical pickup comprising a stem; a substrate mounted on thestem; a laser diode for outputting a laser beam; a photodetectorprovided on the substrate for detecting return light from an object tobe irradiated with the laser beam; a dummy photodetecting regionprovided on the substrate adjacent to the photodetector; a dummyelectrode formed in the dummy photodetecting region so as to surroundthe photodetector; and wiring for connecting the dummy electrode to aground potential.

Preferably, the dummy photodetecting region comprises a first dummyphotodetecting region provided on the substrate adjacent to aphotodetector for detecting a regenerative signal, and a second dummyphotodetecting region provided on the substrate adjacent to aphotodetector for detecting a servo signal. The optical pickup furthercomprises a light shielding film having light absorptivity formed on thesubstrate so as to cover at least the first and second dummyphotodetecting regions. Preferably, the light shielding film comprises apolyimide film.

In accordance with a still further aspect of the present invention,there is provided an optical storage device capable of at least readinginformation stored in an optical storage medium, comprising a base; acarriage movable along the optical storage medium; a stem mounted on thebase; a substrate mounted on the stem; a laser diode for outputting alaser beam; an objective lens mounted on the carriage for focusing thelaser beam from the laser diode onto the optical storage medium; aphotodetector for detecting at least a regenerative signal from returnlight from the optical storage medium; a dummy photodetecting regionprovided on the substrate adjacent to the photodetector; a dummyelectrode formed in the dummy photodetecting region so as to surroundthe photodetector; and wiring for connecting the dummy electrode to aground potential.

In accordance with a still further aspect of the present invention,there is provided an optical pickup comprising a stem; a substratemounted on the stem; a laser diode for outputting a laser beam; aphotodetector provided on the substrate for detecting return light froman object to be irradiated with the laser beam; and a metal layerprovided on the substrate so as to cover at least a region adjacent tothe photodetector, the metal layer having a surface modified so as tohave light absorptivity.

Preferably, the metal layer comprises an anodized aluminum film.

In accordance with a still further aspect of the present invention,there is provided an optical storage device capable of at least readinginformation stored in an optical storage medium, comprising a base; acarriage movable along the optical storage medium; a stem mounted on thebase; a substrate mounted on the stem; a laser diode for outputting alaser beam; an objective lens mounted on the carriage for focusing thelaser beam from the laser diode onto the optical storage medium; aphotodetector provided on said substrate for detecting at least aregenerative signal from a reflected beam from the optical storagemedium; and a metal layer provided on the substrate so as to cover atleast a region adjacent to the photodetector, the metal layer having asurface modified so as to have light absorptivity.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a magneto-optical disk drive;

FIG. 2 is a top plan view of the magneto-optical disk drive;

FIG. 3 is a bottom plan view of a movable optical assembly in themagneto-optical disk drive;

FIG. 4 is a perspective view of the movable optical assembly;

FIG. 5 is a schematic illustration of an optical pickup according to apreferred embodiment of the present invention;

FIG. 6 is an elevational view of an optical unit according to a firstpreferred embodiment;

FIG. 7 is a top plan view of the optical unit shown in FIG. 6;

FIG. 8 is an enlarged sectional view of an LD chip mounting portion inthe optical unit;

FIG. 9 is a top plan view of an optical unit according to a secondpreferred embodiment;

FIG. 10 is a top plan view of an optical unit according to a thirdpreferred embodiment;

FIG. 11 is an elevational view of the optical unit shown in FIG. 10; and

FIG. 12 is a top plan view of an optical unit according to a fourthpreferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown an exploded perspective view of amagneto-optical disk drive 2 according to a preferred embodiment of thepresent invention. The magneto-optical disk drive 2 is composedgenerally of a load/eject mechanism unit 4 and a read/write mechanismunit 6. The load/eject mechanism unit 4 includes a chassis 8 having abottom plate 8 a and a pair of side plates 8 b, and a cartridge holder10 mounted on the chassis 8 so as to be vertically movable with respectto the bottom plate 8 a of the chassis 8.

An insert opening 12 for allowing insertion of a magneto-optical diskcartridge in the direction shown by an arrow C is defined by thecartridge holder 10 and the bottom plate 8 a of the chassis 8. Thecartridge holder 10 is formed with a guide groove 14. The guide groove14 is composed of a first portion obliquely extending from one end ofthe insert opening 12 laterally inward of the cartridge holder 10 and asecond portion extending from an inward end of the first portion to therear end of the cartridge holder 10 in parallel to the longitudinaldirection of the cartridge holder 10. A first slider 16 and a secondslider 18 are slidably engaged with the guide groove 14.

A continuous slit 23 is formed at one side portion of the cartridgeholder 10 to thereby form a first spring arm 20 and a second spring arm22 integral with the cartridge holder 10. Similarly, a slit 25 is formedat the other side portion of the cartridge holder 10 to thereby form athird spring arm 24 integral with the cartridge holder 10. A biasmagnetic field generating device 26 is mounted on the cartridge holder10.

A cartridge identification sensor 28 for detecting a write protectedcondition of the cartridge and the kind of the cartridge is mounted onthe bottom plate 8 a of the chassis 8. Further, an eject motor 32 forejecting the magneto-optical disk cartridge inserted in the cartridgeholder 10 is mounted on the bottom plate 8 a at its rear end portionopposite to the insert opening 12.

Although not shown, a vertically moving mechanism for the cartridgeholder 10 is provided between the chassis 8 and the cartridge holder 10.When the magneto-optical disk cartridge is fully inserted into thecartridge holder 10, the cartridge holder 10 is moved toward the bottomplate 8 a of the chassis 8 by the vertically moving mechanism, so thatthe magneto-optical disk cartridge is pressed on the bottom plate 8 a.In this condition, a shutter of the magneto-optical disk cartridge isopened, and a magneto-optical disk (to be hereinafter described) exposedis chucked to a spindle motor (to be hereinafter described). Such avertically moving mechanism for the cartridge holder 10 is known in theart, so any further description thereof will be omitted herein.

The load/eject mechanism 4 is provided with a flexible printed circuitboard (FPC) 30. The FPC 30 is branched at its front end portion intothree parts, i.e., a first FPC 30 a, a second FPC 30 b, and a third FPC30 c. The first FPC 30 a is connected to the bias magnetic fieldgenerating device 26. The second FPC 30 b is connected to the ejectmotor 32. The third FPC 30 c is connected to the cartridgeidentification sensor 28. The read/write mechanism unit 6 includes abase 34 formed of metal. The load/eject mechanism unit 4 is mounted onthe base 34. A spindle motor 36 is fixed to the base 34.

A movable optical assembly 38, a fixed optical assembly 40, and aprinted circuit board 42 are mounted on the base 34. The movable opticalassembly 38 includes a carriage 44 on which an objective lens 46 ismounted. A connector 48 to be connected to a printed circuit board (notshown) mounted on the upper side of the load/write mechanism unit 4 ismounted on the printed circuit board 42. Reference numeral 50 denotes anFPC for transferring a signal to the spindle motor 36 and a signal tothe movable optical assembly 38. FIG. 2 is a plan view showing anassembled condition of the magneto-optical disk drive 2 shown in FIG. 1.

As shown in FIG. 3, the movable optical assembly 38 is a preassemblyconstructed by inserting first and second guide rails 52 and 54 and twocenter yokes 60 through the carriage 44, and fixing a side yoke 58 toeach center yoke 60. A permanent magnet 62 is fixed to each side yoke58. Thus, two magnetic circuits 56 are formed by the two side yokes 58,the two center yokes 60, and the two permanent magnets 62. A pair ofcoils (not shown) are mounted on the carriage 44, and these coils areconnected to an FPC 64.

These magnetic circuits 56 and coils constitute a voice coil motor(VCM). The VCM is driven by supplying a current through the FPC 64 tothe coils, so that the carriage 44 is moved along the first and secondguide rails 52 and 54. While the carriage 44 is linearly driven by theVCM in this preferred embodiment, an arm to be rotationally driven maybe used in place of the carriage 44, so as to move a light beam acrossthe tracks on an optical storage medium.

Referring to FIG. 4, there is shown a perspective view of the movableoptical assembly 38 in relation to a magneto-optical disk 70. Theobjective lens 46 is mounted on the carriage 44. A pair of coils 72 arefixed to the opposite sides of the carriage 44. Each coil 72 is insertedin a gap defined between the corresponding center yoke 60 and thecorresponding permanent magnet 62.

Referring to FIG. 5, there is shown a schematic illustration of anoptical pickup according to a preferred embodiment of the presentinvention. The optical pickup is configured by the movable opticalassembly 38 having the objective lens 46 and the fixed optical assembly40 mounted on the base 34 shown in FIG. 1. The fixed optical assembly 40includes a stem (optical base) 74 to be mounted on the base 34. The stem74 is formed by pressing a metal plate such as an iron plate. The uppersurface of the stem 74 is plated with gold.

A silicon (Si) substrate 76 is mounted on the stem 74. An SiO₂ film 77is formed on the lower surface of the Si substrate 76 to insulate the Sisubstrate 76 from the stem 74. The resistivity of the Si substrate 76 is10¹⁵ Ωcm, and it is a semiconductor substrate. Any other semiconductorsubstrates such as a germanium (Ge) substrate and a GaAs substrate maybe used in place of the Si substrate 76.

The. SiO₂ film 77 is formed by thermal oxidation of silicon, i.e., byheating the lower surface of a Si wafer. Accordingly, no specialinsulating member is required, and the SiO₂ film 77 can be formed at alow cost. The SiO₂ film 77 may be replaced by any other dielectric filmshaving good heat conductivity, such as aluminum nitride (AlN) andsilicon carbide (SiC). The Si substrate 76 is bonded to the stem 74 byusing Au—Sn solder. To improve the bonding property of the Si substrate76 to the stem 74, the upper surface of the stem 74 is plated with goldas mentioned above. Similarly, the lower surface of the Si substrate 76is also plated with gold.

Referring to FIG. 7, there is shown a plan view of an optical unit 75according to a first preferred embodiment. The optical unit 75 includesthe stem 74 and the Si substrate 76 mounted on the stem 74. The Sisubstrate 76 is integrally formed with a PIN-photodiode 78 for detectinga magneto-optical signal (MO signal), PIN-photodiodes 80 a and 80 b fordetecting a focusing error signal, PIN-photodiodes 82 a and 82 b fordetecting a tracking error signal, and a PIN-photodiode 84 formonitoring the power of a laser diode (LD) chip 86.

Aluminum wiring for leading out the signals from the PIN-photodiodes 78,80 a, 80 b, 82 a, 82 b, and 84 is patterned on the Si substrate 76. TheLD chip 86 is mounted on the Si substrate 76. The LD chip 86 has acentral oscillation wavelength of 685 nm and a beam spread angle ofabout 20°. The LD chip 86 is a chip cut from a wafer. Reference numeral88 denotes an electrode formed on the lower surface of the LD chip 86.The electrode 88 is connected to a terminal 96 by a wire 92. Anelectrode 90 formed on the upper surface of the LD chip 86 is connectedto a terminal 98 by a wire 94.

The PIN-photodiode 78 for detection of a MO signal is connected toterminals 104 and 106 respectively by wires 100 and 102. ThePIN-photodiode 80 a for detection of a focusing error signal isconnected to terminals 112 and 114 respectively by wires 108 and 110.Similarly, the PIN-photodiode 80 b for detection of a focusing errorsignal is connected to terminals 120 and 122 respectively by wires 116and 118. The PIN-photodiode 82 a for detection of a tracking errorsignal is connected to a terminal 126 by a wire 124. Similarly, thePIN-photodiode 82 b for detection of a tracking error signal isconnected to a terminal 130 by a wire 128. The PIN-photodiode 84 fordetection of monitor light is connected to a terminal 134 by a wire 132.These terminals 96, 98, 104, 106, 112, 114, 120, 122, 126, 130, and 134are hermetically sealed with glass and thereby insulated from the stem74.

Referring again to FIG. 5, a laser beam is output from the LD chip 86 inthe horizontal direction, and a reflection prism 140 for reflecting thelaser beam in the vertical direction is therefore mounted on the Sisubstrate 76. The reflection prism 140 is formed of BK7 glass(manufactured by Shott Inc.), and has a reflection surface coated with aphaseless reflecting film. The reflection prism 140 is fabricated byglass molding, polishing, etc.

A cap 142 formed of Kovar is welded to the stem 74. The Si substrate 76and the LD chip 86 are accommodated in the cap 142. The cap 142 has anopening 143 for forming a reciprocative optical path of a light beam.The opening 143 is closed by a glass plate 144. Accordingly, the insideof the cap 142 is enclosed. A hologram 160 formed on the lower surfaceof a glass substrate 162 is bonded by adhesive to the upper surface ofthe cap 142. Mass production of the hologram 160 can be made by forminga plurality of hologram patterns on a single glass substrate by etchingand separating these hologram patterns by dicing to obtain individualholograms.

The hologram 160 has a patterned diffraction grating for dividing afocusing error signal and a tracking error signal. A reflected beam fromthe magneto-optical disk 70 is diffracted by the diffraction grating ofthe hologram 160, and then divisionally focused on the PIN-photodiodes80 a and 80 b for detection of a focusing error signal and on thePIN-photodiodes 82 a and 82 b for detection of a tracking error signal,formed on the Si substrate 76.

A beam splitter unit 146 is fixedly mounted on the glass substrate 162opposite to the hologram 160 by optical adhesive. The beam splitter unit146 includes a glass block 148 having a cylindrical surface 150 and aninclined surface 152, and a glass block 154 bonded by adhesive to theglass block 148. A polarizing light spitting film 151 is formed on thecylindrical surface 151 of the glass block 148. The glass block 154 hasa concave cylindrical surface 156 just fitted with the cylindricalsurface 150, and a cylindrical reflection surface 158. The concavecylindrical surface 156 of the glass block 154 is bonded by opticaladhesive to the cylindrical surface 150 of the glass block 148.

Preferably, the inclined surface 152 of the glass block 148 and thecylindrical reflection surface 158 of the glass block 154 are coatedwith a reflecting film. A glass plate 166 formed with a focusinghologram lens 164 is bonded by adhesive to the lower surface of theglass block 148. Further, a Wollaston prism 168 for splitting thereflected beam into a P-polarized light component and an S-polarizedlight component is bonded by adhesive to the lower surface of the glassblock 154.

The LD chip 86 is bonded by Pb—Sn solder to the Si substrate 76. Thetemperature of soldering of the LD chip 86 to the Si substrate 76 mustbe lower than the temperature of soldering of the Si substrate 76 to thestem 74. Accordingly, Au—Sn solder is used to bond the Si substrate 76to the stem 74 and heated to about 320° C. On the other hand, Pb—Snsolder is used to bond the LD chip 86 to the Si substrate 76 and heatedto about 240° C. Reference numeral 170 denotes a collimator lens formedfrom a normal glass lens. The collimator lens 170 has a focal length of10 mm, and it is an aspherical lens in which the focal length iscorrected by an amount corresponding to the total thickness of the beamsplitter unit 146 and the hologram substrate 162.

Referring to FIG. 8, there is shown an enlarged sectional view of an LDchip mounting portion. An SiO₂ film 176 is formed on the upper surfaceof the Si substrate 76, and a conductor film 178 of a gold plating filmis formed on the SiO₂ film 176. An SiO₂ film 180 is formed on theconductor film 178, and the LD electrode 88 of a gold plating film isformed on the SiO₂ film 180. The SiO₂ films 176 and 180 are formed bysputtering, CVD, or electron beam process.

However, the SiO₂ film 180 is formed selectively on a part of theconductor film 178 to be required for bonding to the LD chip 86 ratherthan the entire surface of the conductor film 178, in consideration ofwire connection to the outside of the LD chip 86. A gold plating film172 is formed on the lower surface of the LD chip 86. The gold platingfilm 172 and the LD electrode 88 are bonded together by Pb—Sn solder174. Referring again to FIG. 7, the conductor film 178 is connected tothe stem 74 by a wire 182. Accordingly, the conductor film 178 is at thesame electric potential as that of the stem 74. Since the stem 74 ismounted on the base 34 of the magneto-optical disk drive, the conductorfilm 178 is at a ground potential.

In operation, a P-polarized laser beam output from the LD chip 86 isreflected by the reflection prism 140 to change its optical path fromthe horizontal direction to the vertical direction. The laser beamreflected by the reflection prism 140 is passed through the hologram 160and then passed through the polarizing light splitting film 151 with atransmittance of about 70%. The laser beam reflected by the polarizinglight splitting film 151 is reflected by the inclined surface 152 andthen focused onto the PIN-photodiode 84 for detection of monitor lightby the focusing hologram lens 164. Then, the power of the LD chip 86 iscontrolled to a predetermined level according to an output signal fromthe PIN-photodiode 84.

The laser beam passed through the polarizing light splitting film 151 isconverted into a collimated beam by the collimator lens 170, and thecollimated beam is focused onto the magneto-optical disk 70 by theobjective lens 46. A reflected beam from the surface of themagneto-optical disk 70 undergoes Kerr rotation according to informationwritten on the magneto-optical disk 70 to thereby include an S-polarizedlight component. The reflected beam is reconverted into a collimatedbeam by the objective lens 46 and then converged by the collimator lens170 to enter the beam splitter unit 146.

The P-polarized light component in the reflected beam is passed throughthe polarizing light splitting film 151 with a transmittance of about70%, and about 30% of the P-polarized light component is reflected bythe polarizing light splitting film 151. On the other hand, theS-polarized light component in the reflected beam is reflected by thepolarizing light splitting film 151 with a reflectance of about 97%.Although the proportion of the S-polarized light component in thereflected beam is very small, the proportion of the S-polarized lightcomponent can be increased by reflecting most of the S-polarized lightcomponent on the polarizing light splitting film 151.

The beam reflected by the polarizing light splitting film 151 is totallyreflected downward by the cylindrical reflection surface 158 of theglass block 154 to enter the Wollaston prism 168. The beam is then splitinto a P-polarized light component and an S-polarized light component bythe Wollaston prism 168, and these components are detected by thePIN-photodiode 78. The PIN-photodiode 78 includes a photodiode fordetecting a P-polarized light component and a photodiode for detectingan S-polarized light component. Signals detected by these twophotodiodes are subjected to differential detection by a method wellknown in the art to thereby detect a magneto-optical signal. On theother hand, the reflected beam transmitted by the polarizing lightsplitting film 151 enters the hologram 160 to undergo diffraction. Thediffracted beams from the hologram 160 enter the PIN-photodiodes 80 aand 80 b for detection of a focusing error signal and thePIN-photodiodes 82 a and 82 b for detection of a tracking error signal.

The hologram 160 may be fabricated by direct drawing using an electronbeam or a laser beam. While it is necessary to tilt a hologram patternto expect high efficiency in the direct drawing, such a requirement canbe met by multiple drawing. Another fabrication method for a hologramincludes the steps of preliminarily directly drawing a large hologrampattern, reducing the hologram pattern by using a stepper to prepare amask, and transferring the hologram pattern by a photolithography. Inthis case, the hologram pattern is fabricated by ion beam etching usinga photoresist or the like as a mask.

According to the optical pickup of the above preferred embodiment,fluctuations in oscillation wavelength due to temperature changes as theemission characteristic of the LD chip 86 can be reduced to reduce theinfluence of chromatic aberration of an optical system. Further, sincethe Si substrate 76 is insulated from the stem 74 by the SiO₂ film 77, adeterioration in radiation characteristic can be reduced. Further, theconductor film 178 is provided between the LD chip 86 and the Sisubstrate 76, and the conductor film 178 is set at the same electricpotential as a ground potential. Accordingly, it is possible to avoidthe crosstalk between a drive signal to the LD chip 86 in writing dataand output signals from the PIN-photodiodes 80 a and 80 b for detectionof a focusing error signal, the PIN-photodiodes 82 a and 82 b fordetection of a tracking error signal, and the PIN-photodiode 84 fordetection of monitor light.

Further, since the potential of the conductor film 178 present under theLD chip 86 is set to a ground potential, the emission characteristic ofthe LD chip 86 can be improved. Further, since the electrodes 88 and 90of the LD chip 86 are connected directly to the terminals 96 and 98provided on the stem 74 by the wires 92 and 94, respectively, possiblerunaround of a signal to each PIN-photodiode can be avoided.

Referring to FIG. 9, there is shown a plan view of an optical unit 75Aaccording to a second preferred embodiment of the present invention. Inthe following description of the second preferred embodiment andsubsequent preferred embodiments, substantially the same parts as thoseof the optical unit 75 according to the first preferred embodiment shownin FIGS. 6 and 7 will be denoted by the same reference numerals, and thedescription thereof will be omitted to avoid repetition.

A dummy photodetecting region 186 is formed adjacent to thePIN-photodiode 78 for detection of a MO signal on the Si substrate 76. Adummy photodetecting region 190 is formed adjacent to thePIN-photodiodes 80 a and 82 a on the Si substrate 76. A dummyphotodetecting region 194 is formed adjacent to the PIN-photodiodes 80 band 82 b on the Si substrate 76. Further, a dummy photodetecting region198 is formed adjacent to the PIN-photodiode 84 for detection of monitorlight on the Si substrate 76. These dummy photodetecting regions 186,190, 194, and 198 function as PIN-photodiodes.

The dummy photodetecting region 186 is formed with a dummy electrode 188surrounding the PIN-photodiode 78. The dummy photodetecting region 190is formed with a dummy electrode 192 surrounding the PIN-photodiodes 80a and 82 a. The dummy photodetecting region 194 is formed with a dummyelectrode 196 surrounding the PIN-photodiodes 80 b and 82 b. The dummyphotodetecting-region 198 is formed with a dummy electrode 200surrounding the PIN-photodiode 84.

The dummy electrode 200 is electrically connected to a terminal 202, andthe terminal 202 is electrically connected to the stem 74. The dummyphotodetecting regions 194 and 196 are electrically connected byaluminum wiring 206. The dummy photodetecting regions 186 and 190 areelectrically connected by aluminum wiring 208. The dummy photodetectingregions 190 and 198 are electrically connected by aluminum wiring 210.Accordingly, all of the dummy photodetecting regions 194, 186, 190, and198 are electrically connected to the stem 74 by the wire 204 toestablish the same electric potential as a ground potential.

The dummy electrodes 188, 192, 196, and 200 are located on the dummyphotodetecting regions 186, 190, 194, and 198 so as to surround thePIN-photodiodes 78, 80 a and 82 a, 80 b and 82 b, and 84, respectively.Accordingly, electrical charge generated in each dummy photodetectingregion does not leak into the electrode of the adjacent PIN-photodiode,but flows into the corresponding dummy electrode. Since each dummyelectrode is electrically connected to the stem 74 having a groundpotential, there is no adverse effect on the signal detected by eachPIN-photodiode.

In the case that the intensity of stray light in recording or erasinginformation, or in the case that an optical modulation rate is high,there is a possibility that the electrical charge in each dummyphotodetecting region may be reduced in responsiveness to leak into anMO signal and a servo signal, for example. To prevent this possibilityand further improve the quality of a signal current, there is providedan optical unit 75B according to a third preferred embodiment of thepresent invention as shown in FIGS. 10 and 11. More specifically, theoptical unit 75B includes a light shielding film 212 having lightabsorptivity formed on the Si substrate 76 except the mounting portionsfor the LD chip 86, the reflection prism 140, and the PIN-photodiodes78, 80 a, 80 b, 82 a, 82 b, and 84 and also except the pad portions forconnection of the aluminum wiring.

Preferably, the light shielding film 212 is formed from a polyimidefilm. The thickness of the polyimide film is about 1 to 5 μm, preferablyabout 2 to 3 μm. By adopting a polyimide film as the light shieldingfilm 212, it can be easily patterned by applying a resist on thesubstrate and next performing exposure and development in the photodiodefabrication step. Alternatively, the light shielding film 212 may beformed of a resist material or the like. Further, the light shieldingfilm 212 hardly contains gas discharging components, so that there isalmost no possibility of contamination of the PIN-photodiodes 78, 80 a,80 b, 82 a, 82 b, and 84 in hermetically sealing the cap 142. Further, acurrent generating stray light can be suppressed to thereby suppressfluctuations in supply voltage and contribute to a reduction in powerconsumption.

While the light shielding film 212 is formed on the Si substrate 76except the above-mentioned portions in the preferred embodiment shown inFIG. 10, the light shielding film in the present invention may be formedso as to cover at least the dummy photodetecting regions 186, 190, 194,and 198. The stray light is caused by the incidence of surfacereflection light from the hologram 160 and the beam splitter unit 146into the optical unit 75B. However, since the light shielding film 212having light absorptivity is present in the optical unit 75B, aphotocurrent is not induced by the stray light except from thePIN-photodiodes 78, 80 a, 80 b, 82 a, 82 b, and 84. Further, there is nopossibility that the stray light may be reflected on the light shieldingfilm 212 to become a new stray light component. As a result, undue noisecomponents are not mixed into a magneto-optical signal current or aservo signal current, thereby obtaining a high-quality output signal.

Referring to FIG. 12, there is shown a plan view of an optical unit 75Caccording to a fourth preferred embodiment of the present invention. Inthe optical unit 75C shown in FIG. 12, the dummy photodetecting regions186, 190, 194, and 198 shown in FIGS. 9 and 10 are not provided, but ametal film 214 having a surface modified so as to have lightabsorptivity is formed on the Si substrate 76. The thickness of themetal film 214 is set to 2 to 8 μm, preferably 4 to 6 μm so as not totransmit light. Preferably, the metal film 214 is formed of the samematerial as that of the wiring patterned on the Si substrate 76. In thiscase, the metal film 214 can be formed in the same process as the wiringforming process, thus improving the workability. Usually, the wiringpattern is formed of aluminum, so that the metal film 214 is preferablyformed of aluminum.

In the case that the metal film 214 is formed of aluminum, the surfaceof the aluminum film is preferably modified to prevent the reflection ofstray light and to have light absorptivity, thereby avoiding reflectedscattering light. More specifically, the surface modification isperformed by masking the surface of the Si substrate 76 so that only alight shielding region (Al film) is exposed, and next anodizing thesurface of the light shielding aluminum film to form a black coating,thereby obtaining a light absorbing surface. The aluminum film thushaving an anodized surface is a light shielding film improved in heatresistance and reliability, so that the stray light component enteringthe Si substrate 76 can be almost cut off and the reflected light fromthe surface of the metal film 214 can also be suppressed. As a result,it is also possible to suppress the generation of a new stray lightcomponent due to re-reflection of reflected scattering light from thesurface of the metal film 214 inside the optical unit 75C. The materialof the metal film 214 is not limited to aluminum, but any othermaterials having light absorptivity may be used.

While the metal film 214 is formed on the almost entire surface of theSi substrate 76 in the preferred embodiment shown in FIG. 12, the metalfilm 214 may be formed on only regions adjacent to the PIN-photodiodes78, 80 a, 80 b, 82 a, 82 b, and 84.

Having thus described the optical pickup of the present invention inrelation to a magneto-optical disk drive, the application of the presentinvention is not limited to the magneto-optical disk drive. For example,the optical pickup of the present invention is applicable also to anyother types of optical storage devices using an optical pickup fordriving an optical storage medium such as CD, DVD, and optical card.Further, the optical pickup of the present invention is applicable alsoto a microscope unit and various inspection devices, for example.

According to the present invention as described above, the substrateintegrally formed with a plurality of photodiodes is mounted on the stemwith an insulating film being formed on the lower surface (mountsurface) of the substrate, so that the radiation characteristic of theLD chip mounted on the upper surface of the substrate can be improved tothereby obtain a stable emission characteristic. Further, a conductorfilm is provided under the lower surface of the LD chip, and theelectric potential of the conductor film is set to a ground potential,thereby reducing the crosstalk between a drive signal to the LD chip andoutput signals from the photodiodes to obtain a good servo signal andregenerative signal. Accordingly, the radiation from the LD chip can befacilitated to allow a stable writing operation, and an LD chip drivesignal component mixing into a servo signal can be suppressed to therebyimprove the quality of the servo signal and allow stable control.Thusly, it is possible to provide an optical pickup which can ensurehigh reliability and low cost.

According to the preferred embodiment employing a dummy photodetectingregion formed adjacent to each photodiode, a photocurrent induced bystray light incident on the dummy photodetecting region can be guardedby a dummy electrode formed in the dummy photodetecting region, therebyavoiding adverse effects on a regenerative signal detector and/or aservo signal detector to obtain a good regenerative signal and servosignal. According to the preferred embodiment employing a lightshielding film or metal film having light absorptivity so formed as tocover the upper surface of the substrate, a stray light componentreflecting on the surface of the film can be suppressed to prevent thegeneration of new stray light. Accordingly, the regenerative signal andthe servo signal can be improved in quality to thereby provide anoptical pickup having high reliability.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

1-24. (canceled)
 25. An optical pickup comprising: a stem; a substratemounted on said stem; a laser diode for outputting a laser beam; aphotodetector provided on said substrate for detecting return light froman object to be irradiated with said laser beam; and a metal layerprovided on said substrate so as to cover at least a region adjacent tosaid photodetector, said metal layer having a surface modified so as tohave light absorptivity.
 26. An optical pickup according to claim 25,wherein said metal layer comprises an anodized aluminum film.
 27. Anoptical storage device capable of at least reading information stored inan optical storage medium, comprising: a base; a carriage movable alongsaid optical storage medium; a stem mounted on said base; a substratemounted on said stem; a laser diode for outputting a laser beam; anobjective lens mounted on said carriage for focusing said laser beamfrom said laser diode onto said optical storage medium; a photodetectorprovided on said substrate for detecting at least a regenerative signalfrom a reflected beam from said optical storage medium; and a metallayer provided on said substrate so as to cover at least a regionadjacent to said photodetector, said metal layer having a surfacemodified so as to have light absorptivity.
 28. An optical storage deviceaccording to claim 27, wherein said metal layer comprises an anodizedaluminum film.